(1) Field of the Invention
The present invention relates to an operational amplifier, and more particularly to an operational amplifier having an improved phase compensating circuit which can prevent high frequency noises on a power supply voltage from being transmitted to an output terminal of the operational amplifier.
(2) Description of the Prior Art
In general, an operational amplifier is constituted mainly by a differential circuit, an output amplifier and a phase compensating circuit. The phase compensating circuit of a conventional operational amplifier comprises a capacitor and a buffer amplifier inserted in a feedback circuit of the output amplifier and prevents the self-oscillation of the operational amplifier by decreasing the high frequency gain thereof.
However, in the conventional operational amplifier, the phase compensating circuit includes the output amplifier in a part of its signal path, and thus the signal delay time of the phase compensating circuit is longer than that of the output amplifier. Since the cut off frequency of a circuit is approximately in inverse proportion to the signal delay time of the circuit, the cut off frequency of the phase compensating circuit is lower than that of the output amplifier. Therefore, in the frequency range between the cut off frequencies of the output amplifier and the phase compensating circuit, the phase compensating circuit does not operate even though the output amplifier operates, i.e., even though the output amplifier has a gain larger than 1. As a result, noise in the above-mentioned frequency range from the operating voltage source of the operational amplifier appear at the output terminal thereof. Therefore, in the conventional operational amplifier, the SVRR, i.e., supply voltage rejection ratio with regard to the high frequency noises on the operating voltage source cannot be small, and the conventional operational amplifier requires high precision ripple filter circuits connected to the operating voltage terminals thereof. Especially in a hybrid type integrated circuit constituted by digital circuits and analog circuits containing at least an operational amplifier, the SVRR of the operational amplifier cannot be small because of the high frequeny noises superposed on the power supply voltage caused by the digital circuits.